Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150° C. and 250° C. into a urea synthesis zone. The resulting urea formation can be presented best in the form of two consecutive reaction steps, in the first step ammonium carbamate being formed according to the exothermic reaction:2NH3+CO2→H2N—CO—ONH4 after which the ammonium carbamate formed is dehydrated in the second step to give urea according to the endothermic equilibrium reaction:H2N—CO—ONH4H2N—CO—NH2+H2O
The extent to which these reactions take place depends among other things on the temperature and the ammonia excess used. The reaction product obtained in a urea synthesis solution substantially consists of urea, water, unbound ammonia and ammonium carbamate. The ammonium carbamate and the ammonia are removed from the solution and are generally returned to the urea synthesis zone. In addition to the above-mentioned solution in the urea synthesis zone, a gas mixture is formed which consists of unconverted ammonia and carbon dioxide together with inert gases, the so called reactor off-gas. The urea synthesis section may comprise separate zones for the formation of ammonium carbamate and urea. These zones may also be combined in a single apparatus.
In a urea stripping plant the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual ammonia excess largely takes place at a pressure that is essentially almost equal to the pressure in the synthesis reactor. This decomposition and expulsion take place in one or more stripper(s) installed downstream of the reactor, possibly with the aid of a stripping gas such as, for example, carbon dioxide and/or ammonia, and with the addition of heat. It is also possible to apply thermal stripping. Thermal stripping means that use is made exclusively of the supply of heat to decompose ammonium carbamate and remove the ammonia and carbon dioxide present from the urea solution. The gas stream leaving a stripper contains ammonia and carbon dioxide which are condensed in a high-pressure condenser and then returned to the urea synthesis zone.
In a urea stripping plant the synthesis zone is operated at a temperature of 160-240° C. and preferably at a temperature of 170-220° C. The pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-20 MPa. In the art, these ranges are generally considered to represent “high pressure” (as also used in connection with a conventional “High Pressure Carbamate Condenser”). The ammonia to carbon dioxide molar ratio (N/C ratio) in the urea synthesis zone of a stripping plant lies usually in between 2.2 and 5 and preferably between 2.5 and 4.5 mol/mol. The synthesis zone can be carried out in a single reactor or in a plurality of reactors arranged in parallel or series.
After the stripping treatment, the pressure of the stripped urea solution is reduced in a urea recovery section. In a recovery section the non-converted ammonia and carbon dioxide in the urea solution is separated from the urea and water solution. A recovery section comprises usually a heater, a liquid/gas separation section and a condenser. The urea solution entering a recovery section is heated to vaporize the volatile components ammonia and carbon dioxide from that solution. The heating agent used in the heater is usually steam. The formed vapor in said heater is separated from the aqueous urea solution in the liquid/gas whereafter said vapor is condensed in the condenser to form a carbamate solution. The released condensation heat is usually dissipated in cooling water. The formed carbamate solution in that recovery section operated at a lower pressure than the pressure in the synthesis section is preferably returned to the urea synthesis section operating at synthesis pressure. The recovery section is generally a single section or can be a plurality of recovery sections arranged in series.
Urea being a bulk chemical, a continuous desire in the art is to seek ways to achieve a more economic operation. Particularly, it is desired to produce urea in a more energy-efficient way. Also, it is desired to increase the conversion rate of the urea synthesis process.
As further background art, reference is made to WO 96/20170. Herein, in a process for urea production, a urea reaction mixture is obtained in a main reaction space. The reaction mixture is subjected to stripping and then sent to a recovery section. A carbamate stream from the recovery section is recycled to an auxiliary reaction space. Therein carbamate is converted to urea. The urea liquid stream from the auxiliary reaction space is sent to the stripper. The entire gas stream from the stripper is sent to the condenser.